Have you got a source on this or anything to back it up? Unless things have changed a great deal in the 7 years since I moved from Canberra, that isn't true at all.
Participation numbers for one, the fact that they draw similar sized crowds to the Raiders average without a local team should be a big warning sign as well. There're others stats as well, google search data and social media statistics and the such.
I was born and raised here, and have lived here for decades, since before the Raiders entered the NSWRL. In all that time each fanbase would swear that their sport was way bigger than the other, but aside from a small period in the 90s the truth has always lay in the middle.
Because of the unique cultural and historical circumstances of Canberra neither sport has dominated the other in the post war era, in fact both are so close in popularity that it's basically impossible to say which is bigger than the other at any one time. The difference in popularity between the two would be so negligible that it's meaningless anyway. There's also way more overlap between the fanbases of both sports in Canberra than you see in the other parts of the country.
The only reason that the Aussie Rules fanbase in Canberra isn't as visible as RL's is because it doesn't have a single entity to get behind like the Raiders. If you gave them a local AFL team it would be as big as, if not slightly bigger than, the Raiders from day one, and anybody claiming otherwise is either astonishingly ignorant or monumentally full of shit.